The amount of transistors in a liquid crystal display (LCD) is approximate over millions. An arrangement of a liquid crystal is controlled through a corresponding transistor and so brightness of each pixel corresponding the transistor is simultaneously determined to display an image. Each transistor connects two conducting lines of a gate line and a data line. The gate line controls a switch of the transistor to make the transistor turn on at a certain time for receiving the image data. At the certain time, the data line delivers data to the transistor. These electrical features of the conducting line and the related line effect a resolution and a response time of the LCD.
Recently, the developing tendency of the LCD tends to become faster in the response time and larger in the size. In general, a large-size (e.g.: larger than 20 inches for TFT-LCD) LCD employs aluminum (Al) as the base material of the conducting line due to the cost of Al is low and the electrical conductivity of Al is good. Mainly, the conducting line of the LCD is made of a single layer of Al, a double layers of Al and another metal, or Al alloy and another metal.
The electrical conductivity of the Al conducting line can't break through the bottleneck of the electrical conductivity in those conventional arts regarding the demand for a larger size LCD with higher resolution and faster response time (especially for LCD-TV). It must find new material to satisfy the demand for higher resolution and faster response time.
Recently, in the semiconductor industry, the Al conducting line is replaced with copper (Cu) having better electrical conductivity for solving the problem of the electrical conductivity. However, no appropriate etching solution for Cu and so to form Cu line is very hard. The damascene process is illustrated as below. First, a hole 20 is formed on a substrate 10 and then a thin and continuous copper seed layer 30 is formed on a surface of the substrate 10, as shown in FIG. 2A. The copper seed layer 30 can raise an adhesive force between the Cu line and the substrate, and benefit a growth of the Cu line in the successive electroplating process. The copper seed layer 30 must simultaneously cover with a surface of the hole 20 and so the copper seed layer 30 can grow along the surface of the hole 20 during the electroplating process. Furthermore, the copper seed layer 30 must be thin, even, and continuous for avoiding to generate some hollows. However, if there is a doubt about a current leakage resulted from the Cu dispersing to other layers, a barrier layer 40 is added for preventing the Cu from diffusing into other layers, as shown in FIG. 2B. The barrier layer also can avoid the Cu reacting with silicon simultaneously. Hence, the barrier layer 40 is formed before the copper seed layer 30 when there is the aforementioned doubt.
The electroplating process is then performed to electroplate a Cu electroplating layer 50 on the copper seed layer 30 for making the Cu electroplating layer 50 cover with the copper seed layer 30 continuously, smoothly and fine. The hole 20 will be filled with the Cu electroplating layer 50 and no hollow is produced, as shown in FIG. 3 (the barrier layer not being shown). Finally, a chemical mechanical polishing (CMP) is performed to polish the copper seed layer 30 until the surface of the substrate 10 and only the portion of the copper seed layer 30 inside of the hole 20 is remained, as shown in FIG. 4. Hence, the forming of the Cu conducting line is finished.
However, the aforementioned process of the Cu conducting line can not be apply to the TFT-LCD. The main reason is that the area of the liquid crystal panel is quite large compared with the 12 inches wafer in semiconductor industry. Therefore, for the forming of the copper seed layer 30, the thickness of the copper seed layer 30 is hard to control within a certain range and so some hollows are easily produced therein. Furthermore, the electroplating rate of the Cu electroplating layer must be controlled more accurately for the electroplating rate being almost equal in every position. But, the difficulties of these problems will increase with the size of the LCD. Hence, for a large-size liquid crystal panel, the problems is hard to overcome.